This invention relates generally to axle driving apparatus and, more particularly, to a return to neutral mechanism adapted to be mounted to the casing of a transaxle such as zero-turn transaxle. The return to neutral mechanism disclosed herein could also be used with hydraulic pumps or other types of hydrostatic transaxies.
Transaxle and hydrostatic transmission assemblies (xe2x80x9cHSTsxe2x80x9d) are known in the art. Generally, an HST includes a center section on which is mounted to a rotating hydraulic pump and a rotating hydraulic motor. The hydraulic pump and the hydraulic motor each carry a plurality of reciprocating pistons which are in fluid communication through hydraulic porting formed in the center section. Rotation of the hydraulic pump against a moveable swash plate creates an axial motion of the pump pistons that forces an operating oil through the hydraulic porting to the hydraulic motor to move the motor pistons. The axial motion of the motor pistons causes the hydraulic motor to rotate as the motor pistons bear against a thrust bearing. In this manner, the rotation of the hydraulic motor may be used to drive the vehicle axles of a riding lawn mower, small tractor and the like.
To adjust the speed and direction of rotation of the hydraulic motor and, accordingly, the speed and direction of rotation of the vehicle axles, the position of the swash plate with respect to the hydraulic pump pistons may be changed. The orientation with which the swash plate addresses the hydraulic pump pistons can be changed to control whether the hydraulic motor rotates in the forward direction or in the reverse direction. Additionally, the angle at which the swash plate addresses the hydraulic pump pistons can be changed to increase or decrease the amount of operating oil that is forced from the hydraulic pump to the hydraulic motor to change the speed at which the hydraulic motor rotates.
For use in changing the position of the moveable swash plate, it is known to include a trunnion arm that is coupled to the swash plate. A speed change lever or a speed change pedal is, in turn, coupled to the trunnion arm through a wire or other driving link. In this manner, movement of the speed change lever/pedal results in movement of the trunnion arm to change the position of the swash plate to thereby control the speed and direction of the vehicle. Examples of such mechanisms for adjusting the speed of a vehicle may be seen in U.S. Pat. Nos. 6,122,996 and 5,819,537 which are incorporated herein by reference in their entirety.
For placing the swash plate in a position that neither effects the speed nor the direction of rotation of the hydraulic motor, i.e., the neutral position, known hydraulic pumps and hydrostatic transaxles provide a return to neutral mechanism that is normally implemented as an integral part of the vehicle linkage. While these return to neutral mechanisms work for their intended purpose, they do suffer disadvantages. For example, these known return to neutral mechanisms fail to allow for flexibility whereby different types and orientations of driving linkages may be used in connection with the hydraulic pumps and hydraulic transaxles.
To overcome these disadvantages, the present invention is realized in an improved speed adjusting mechanism having an integral return to neutral mechanism that is adapted to be mounted to the casing of a hydraulic pump or hydrostatic transaxle. For simplicity of explanation the invention will be described in association with an integrated zero turn transaxle (IZT). The IZT includes a hydraulic transmission mounted within the casing that includes a rotatable hydraulic pump in fluid communication with a rotatable hydraulic motor and a moveable swash plate cooperable with the rotatable hydraulic pump for controlling the speed and direction of rotation of the hydraulic motor. The rotation of the hydraulic motor is used to drive a single axle shaft.
For controlling the positioning of the swash plate, the transaxle further includes a rotatable trunnion arm coupled to the moveable swash plate. The rotatable trunnion arm extends from the casing and is coupled to the speed adjusting mechanism. The speed adjusting mechanism is mounted to the casing and is used to rotate the trunnion arm to change the orientation of the swash plate to change the speed and direction of rotation of the hydraulic motor.
More specifically, the speed adjusting mechanism includes a return arm adapted to be mounted to the casing in a fixed position indicative of a neutral position of the trunnion arm. The neutral position of the trunnion arm is the position of the trunnion arm in which the swash plate does not influence the speed and direction of rotation of the hydraulic motor. Additionally, the speed adjusting mechanism includes a control arm that is mounted to and moves the trunnion arm. A pair of scissor return arms are provided that are adapted to move the control arm in cooperation with the return arm for the purpose of moving the trunnion arm to the neutral position. To provide additional mounting flexibility, the control arm is adapted to be mounted to the trunnion arm in any one of a plurality of different positions and the return arm is capable of being mounted to the casing in a corresponding position such that the return arm can be aligned with the control arm to establish the neutral position.
In a further embodiment of this invention, the return to neutral feature is unidirectional, in that it provides a return force when the unit is stroked in one direction, either reverse or forward, but does not provide any return force when the unit is stroked in the opposite direction. In certain applications, the user may want to have such a return force only when the unit is in reverse, for operational purposes, but not want to have it in the forward direction, since the maintenance of the force needed to overcome the return force may be tiring to the user, or may be otherwise unnecessary.
A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative embodiment and which are indicative of the various ways in which the principles of the invention may be employed.